Richmond: Presumptive Republican presidential nominee Donald Trump has slammed US President Barack Obama for endorsing Hillary Clinton as the Democratic candidate despite being under "criminal investigation".

Trump, however, said he was ready to face Clinton in the November elections, which he claimed would attract the largest ever voter turnout.

"You have a President coming out and endorsing somebody who is under criminal investigation. Is this supposed to be the way the country supposed to be?" Trump told his cheering supporters at an election rally in Richmond, Virginia yesterday.

This was Trump's first public rally since Obama endorsed Clinton as Democratic presidential nominee against Senator Bernie Sanders from Vermont.

The rally at spacious Richmond Coliseum attracted several thousands people. But the arena having a capacity of 13,500 was more than half empty.

Trump said the rally was organised at a short notice of less than one day.

The real estate mogul from New York said he would make a major policy speech on Clinton in New Hampshire on Monday. To the cheering of his supporters, Trump described this as a "crooked Hillary" speech.

"There is so much corruption. If the system works, she is not allowed to run. It is really unfair for the other people," Trump said referring to a latest news report that an Indian American Rajiv Fernando was appointed to the important International Security Advisory board because of his alleged massive donations to the Clinton Foundation.

The Advisory Board advises the State Department on nuclear weapons and other issues of national security. Based out of Chicago, Fernando is a securities trader president of Chopper Trading.

According to ABC News, Fernando was a major bundler for the Obama Campaign, raising more than USD 500,000 for his re-election cycle. And before his appointment at the State Department, he gave between USD100,000 and USD250,000 to the William J Clinton Foundation.

Trump cited this as another example for corruption by Hillary Clinton. "He made a contribution of USD 250,000 and all of a sudden he's on this very important and vital board," he said.

Reiterating that Clinton should not be allowed to run for the elections, he said he is ready to run against the former US Secretary of State and exuded confidence of winning the election.